What are planetary hours?

Ancient astronomers counted seven “planets”, meaning wandering lights against the fixed stars: Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon. Ranked from the slowest to the fastest, as they appear from the Earth, they form what is now called the Chaldean order. A planetary hour is simply an hour assigned to one of these seven, taken in that order and repeated around the clock.

The hours are unequal, or “temporal”. Before mechanical clocks spread through Europe in the fourteenth century, an hour usually meant one twelfth of the daylight or one twelfth of the night, so a summer day-hour could be far longer than a winter one. Planetary hours keep that older reckoning, which is why they cannot be read off an ordinary clock.

EvidenceThe hours and the week

The same arithmetic explains the names of the weekdays. Writing in the early third century CE, the Roman historian Cassius Dio describes the seven-day planetary week as a custom that had spread through the Roman world, which he attributes to the Egyptians. One of the explanations he gives is the hour count: give the first hour of a day to Saturn and the following hours to each planet in the Chaldean order, and the first hour of the next day falls to the Sun, the next to the Moon, and so on (Roman History 37.18–19). The result is the planetary week: Saturn’s day, the Sun’s day, the Moon’s day, then Mars, Mercury, Jupiter and Venus, which survive in Saturday, Sunday and Monday and in the Romance names of the other days.

A worked example

Take a Wednesday with sunrise at 05:00, sunset at 21:00 and the next sunrise at 05:00. Daylight lasts sixteen hours, so each day-hour lasts 16 × 60 ÷ 12 = 80 minutes. The night lasts eight hours, so each night-hour lasts 40 minutes. Wednesday belongs to Mercury, so the first hour, 05:00 to 06:20, is Mercury’s; the second, 06:20 to 07:40, goes to the Moon; the third to Saturn, and so on. The night carries straight on: the twelfth day-hour, 19:40 to 21:00, is Mars’s, so the first night-hour, from 21:00 to 21:40, belongs to the Sun. At the equinoxes day and night are roughly equal and every planetary hour lasts about sixty minutes.

ModernWhy calculators disagree by a few minutes

Sunrise can be defined in more than one way: the moment the upper edge of the Sun clears the horizon, the moment its centre does, with or without an allowance for atmospheric refraction, or by a local horizon with hills in it. Different apps and almanacs make different choices, and each choice moves every hour boundary of the day. The old texts did not specify any of this. The calculator below uses the common almanac convention (upper edge, standard refraction), which usually agrees with published sunrise tables to within a minute or two.

Near the poles the system breaks down: when the Sun does not set, or does not rise, there is no daylight to divide. The calculator will say so rather than guess.

Late medieval and Renaissance ritual texts attached an angel to each planet, and so to each day. The best-known list is in the Heptameron, a short handbook falsely attributed to the thirteenth-century physician Peter of Abano and first printed in 1559. It names a ruling angel for each weekday, and it tells the operator to work on the day, and in the hour, of the planet concerned.

TraditionHow the grimoires used the hours

The planetary hour was a door in time. The Arabic Ghāyat al-Ḥakīm, known in Latin as the Picatrix, tells the maker of a talisman to begin on the planet’s day and in its hour, with the planet strong in the sky. The Heptameron arranges its conjurations by weekday and hour. The Ars Paulina, the third book of the seventeenth-century Lemegeton, goes further and gives a ruling angel to each of the twenty-four hours. The Archive traces these texts in more detail in Astral Magic & the Planetary Hours and The Orders of Angels.

Evidence and Modern What can and cannot be said

The history of the system is well documented: the Chaldean order, the unequal hours, the derivation of the week and the rules the grimoires give are all in the sources. Whether a planetary hour has any effect on events is a different kind of claim, a matter of religious and magical belief rather than of evidence, and no scientific study supports it. Modern practitioners use the hours as a devotional calendar or a way of structuring attention. Discovery Science apps compute the hours exactly and present the traditions as the texts give them; the binaural sound used in several of the apps is a modern addition, not part of the historical rites.